Radio frequency wattmeter



April 7, g H. BROWN 2,278,687

I RADIO FREQUENCY WATTMETER Filed April 2, 1940 1 20. 1. j 15 i7 IL INPU'I'FQ 119 *4 9 21 3 jifauq puq 3nnentor George H Brawn y IK B I I I C(ttorneg Patented Apr. 7, 1942 i UNITED STATES PATENT OFFICE aanro FRE UENCY WATTMETEB.

' George H. Brown, Haddonfleld, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application April 2, 1940, Serial No. 327,400

8 Claims. (Cl. 171-95) Where' a given radio transmitter, for'example,

is connected by a plurality of transmission lines to a number of radiators comprising a directive antenna array, it is desirable to be able to insert a wattmeter in the transmission lines successively without detuning or otherwise aflecting the system. It is, therefore, a further object of this invention to provide a radio frequency wattmeter having a negligible series impedance and a high shunt impedance so that it may be inserted in and removed from a radio circuit whenever necessary without unbalancing the circuit.

Other desirable features which are provided by the wattmeter of the present invention are: that it is direct reading; that it utilizes a direct cur-- rent indicator which may be located at a consid- This invention will be better understood from the following description when considered in connection with the accompanying drawing, in which Figure 1 is a schematic diagram of the invention; Figures 2 and 3 are schematic diagrams illustrating the methodof calibrating the meter; and Figures 4, 5 and 6 are vector diagrams for explaining the operation and calibration of the meter. Similar reference numerals refer to 10 similar parts throughout the several drawings.

Referring to Fig. l, the wattmeter is provided with input terminals 9 and output terminals ll. One input and one output terminal is shown grounded, but it is to be understood that this is not necessarily actual ground, but may be the ground of the system. By means of these terminals the meter is inserted in the transmission line supplying power to a load. A conductor II is connected between the ungrounded input and output terminals. The conductor is preferably a copper tube or other low resistance conductor, and is bent so as to form two suitably spaced single turn line current coils I! and I! through which the entire line current flows. The reactance of these coils is negligible at the operating frequency, and the series resistance of the conductor is also negligible, so that the power loss in the meteris likewise negligible.

A pair of current pick-up coils l8 and 2| are erable distance from the meter itself; that it connected in se ies a dadiusta ly coupl d to the may be readily calibrated without reference to another radio frequency meter; that its calibration is independent of frequency; that it operates without appreciable error with a current having two line current coils l5 and II, respectively. The size of the current pick-up coils is not criti-. cal and may be, for example, approximately 10 turns. The outer terminals of the two current a substantial leading or a lagging power factor; Dic -11D coils l9. 2| are connected through th that itis conveniently adapted for use over a wide range of frequencies, which range may easily be extended by changing several coils; that it is extremely economical to build; that no external power supply is required as it uses no vacuum f tubes; and that it consumes a minimum of power from the line.

The foregoing objects are accomplished by energizing a pair of serially connected current responsive devices by two radio frequency currents.

The first current is proportional in amplitude and phase to the current which is supplied to a load. while the second current is similarly proportional to the voltage across the load.. The two curheater elements 21, 29 of a pair of current responsive vacuum thermocouples SI, 3!, respectively, to ground. The circuit described so far is the line current responsive portion of the wattmeter. The line voltage responsive portion will. now be described.

rents are combined so as toproduce an indication minal of the coil is connected to the electrical which is proportional to the power delivered to the load. This invention is a modification of the wattmeter disclosed ina copending application of George H. Brown and Jess'Epstein, Serial No. 312,058, filed January 2, 1940.

mid-point between current pick-up coil I! and 2| through a resistor 51 and a meter II. It is to be understood that these elements may be connected in this circuit in any desired order.

vacuum thermocouples are connected in series opposition, that is, the negative D. C. output terminals, are connected together and a suitable D. C. meter 41 is connected between the two positive terminals. Both terminals of the D. C. meter are preferably by-passed to ground by a pair of capacitors 49, 5|. As a result, the meter deflection is proportional to the difference between the currents flowing through the two thermocouple heaters.

The adjustment and calibration of the wattmeter requires three distinct operations. First, the thermocouple unit is adjusted; second, the current coils are adjusted; and third, the voltage coils are adjusted. These adjustments will now be described.

The thermocouple adjustment and calibration is accomplished with the thermocouple unit 53 disconnected from the remainder of the apparatus. The thermocouple heaters are connected in series to any available source of direct or alternating current, preferably of the operating radio frequency, and the output meter connected between the two positive output terminals, as described above. If the two thermocouples are exactly equal the meter reading will be zero for all values of applied current. This is not likely to be the case at flrst, however, and so it is necessary to adjust one or the other thermocouple until the output is reduced to zero. This is accomplished by connecting a resistor across the couple which is delivering the greater voltage. Such a resistor, 53, is shown in Fig. 1. The unit may then be calibrated to determine its deflection constant K by energizing one of the thermocouples, the meter still being connected across the output of both couples, and determining the ratio between the energizing current and the deflection. It will also be useful to determine the meter constant K when the meter is connected to the output of only one thermocouple and that thermocouple is energized. The unit is then reconnected to the wattmeter.

The current adjustment is accomplished by disconnecting the voltage line inductor 35 from the line and passing an alternating current through the current coils as shown in Fig. 2. The output terminals 1 l are short circuited, or

connected to a given load, and a source of current 55, whose frequency is within the desired operating range, is connected to the input terminals I. The amplitude of current In flowing through the line current coils II and I1 is adiusted to some value which is within the desired operating range of the meter, with reference to the probable load impedance of the device whose power "consumption is to be measured. For example, assume the meter is to read 1 kw. at mid scale. With the output short-circuited, theline current is adiusted to that value which will flow when 1 kw. is being consumed by the assumed load. Then the coupling between the'line current coils I I, I1 and the pickup coils I, II is adjusted until the current I: in the two thermocouples is equal to a value which is determined from Equation 7, which is derived below, assuming any convenient ratio between I1 and I: and unity power factor. Preferably, I1 and I: are made equal. This operation is facilitated by means of a switch I! which connects the meter across either one of the thermocouples. The current and voltage coils should be mounted at right angles to each other, or otherwise shielded.

The voltage calibration is made by applying the f source 55 to the line voltage coil as shown in Fig. 3. No current flows through the conductor is during the voltage adjustment. The line voltage is determined for the assumed load impedance and power, and the generator 55 is adjusted to this value. A certain current will flow through inductor 35 to ground which will induce a voltage in the secondary 39 of the voltage transformer. The transformer secondary, in series with the two current pick-up coils l8 and ii in parallel, constitutes a link circuit which is tuned to resonance by capacitor ll. Keeping this circuit tuned to resonance, the coupling between the voltage transformer primary 35 and secondary 39 is then adjusted with the meter connected to one of the thermocouples until its deflection is equal to the value for I: previously determined from Equation 7. In using Equation 7 for the voltage and current calibrations described above. the meter constant K must be used, as the meter is connected to only one thermocouple. The same meter reading should then be obtained when the meter is connected to the other thermocouple. The path of the current 211 produced by the line voltage is shown in Fig. 3. Since the current 2I1 divides equally between two equal current paths to ground, no resultant voltage is induced in the line current inductors l5 and 11. Consequently, the meter currents produced by the line current and the 'line voltage are independent.

The line voltage inductor 35 is preferably tuned to resonance at the operating frequency to increase the shunt impedance of the meter, but this adjustment will not affect the calibration of the instrument. The meter is then connected as shown in Fig. l and it is ready for use.

The phase relation between the line current 11. and the circulating current I: is illustrated in Fig. 4, to which reference is now made. The line current 1:. is shown leading the line voltage Er. by an angle The voltage E: induced in the line pick-up coils i9 and Ii is made to lead the line current Ir. by by suitably phasing the transformer secondary. The resultant current I: that flows through the thermocouples and inductors i8 and II is inductive and therefore lags the voltage E: by 90'. Consequently, the induced current I: is in phase with the line current 11., a condition which is required.

Referring now to Figs. 3 and 5, the phase relation between the mesh current I1 and the line voltage E1. will be determined. The current IA flowing through the inductor l5 lags the line voltage Ex. by 90". This current Ia induces a quadrature voltage En across the pickup coil 38 and since the link circuit is tuned to series resonance by the capacitor ll, it has unity power factor and the link current 211 is in phase with the induced voltage Es. The mesh current I1. therefore, is also in phase with the line voltage Er... Consequently, it will be seen that the two mesh currents I1 and I: are respectively in phase with the line voltage E1. and line current In.

It-now remains to be shown that the meter deflection is proportional to E111. cos 4 which is the power supplied to the load. The mesh equations may be stated as follows:

{ squaring both sides:

and similarly forthe other mesh:

(] 1- z|)= i+ i 2 005 e The meter deflection is equal to the product of the meter constant K and the thermocouple current squared. Consequently, the deflection DA, due to the current in one mesh, when the meter is connected across both thermocouples, is:

while the deflection Da, due to the current in the other mesh, is;

The total deflection, Dr is equal to the difference, that is:

Equation 7, indicates that the resultant meter deflection is proportional to the product of the currents I1 and I: and their phase angle cos o. Since these currents are respectively in phase with and proportional to the line current and line voltage, the total deflection is proportional to power, as required.

It will be appreciated that the calibration of the meter is independent of frequency; the only adjustment required to shift from one frequency to another is to tune the link circuit to resonance, for which purpose the meter 45 is provided. The mesh currents are independent of frequency changes since any increase in coupling due to an increase in frequency is exactly equalized by a like increase in the mesh impedance. That is:

ZL(:I 1 L l (8) where M1 is the mutual coupling between the line current and line pick-up coils, and L is the mesh circuit inductance, and

ELMRAIQ P L RL where Mr; and Mc are the mutual coupling of the voltage transformer and link circuit transond coupling transformer 25 in the link circuit of the prior device, and the inclusion of resistor 51 in the simplified link circuit. Fewer parts are required in the present arrangement than in the earlier device, and, in addition, there are fewer adjustments to make and fewer to get out of adjustment. Thus the stability of the instrument is also better than that of the more complicated arrangement.

An additional advantage is found in the improved-accuracy over a wide range of frequencies which is made possible by the change. In the original arrangement, it was. necessary to maintain a link current In of the order of 3 amperes 6f calibration between different operating frequencies depended upon the resistance of the link circuit remaining constant. The radio frequency resistance of this link circuit was quite low, varying from 0.5 ohm at 550 kc. to 0.6 ohm at 1500 kc. This meant that if the instrument was calibrated at 1000 kc. it would read 10 percent high at 550 kc. and 10 percent lowat 1500 kc. While several ohms of fixed resistance could be inserted in this circuit to reduce the percent change of resistance, it is apparent that for each ohm inserted the power loss in the meter would be increased by. 9 watts.

In the present circuit, however, the link current is only about 0.3 ampere. The added power loss is less than one-tenth of a watt for each ohm I added. Sufficient resistance may, therefore, be

added to reduce the percent change of resistance with frequency, so that the calibration of the instrument is constant throughout the broadcast frequency range.

I have thus described a radio frequency wattmeter which is accurate and reliable, which has a negligible effect on the circuit with which it is used, which is operable .over a wide range of which requires no external source coupled to said resonant circuit, means effectively connecting said inductor between a point intermediate said pick-up coils and ground, and means for tuning said inductor and said pick-up coils to resonance at the operating frequency.

2. A radio frequency wattmeter comprising coupling means for connection in series with a power circuit energizing a load, pick-up coil means coupled to said coupling means, -a pair of current responsive devices connected in series with said pick-up coil means, means for grounding a point intermediate said series connected devices, high impedance coupling means connected in parallel with said power circuit, and means coupling said high impedance coupling means between a point intermediate the ends 4. A device of the character described in claim 1 which is further characterized by the inclusion of a resistor in said means effectively connecting said inductor between a point intermediate said pick-up coils and ground.

in order to insure sufiicient mesh current with- 5. A radio frequency'wattmeter comprising inductor means adapted to be connected in series with a power circuit energizing a load, a pair of pick-up coils coupled to said inductor means, a -pair of current responsive devices, connections from said devices to said pick-up coils, means coupled to saidpower circuit for deriving 9. current in phase with the voltage of said line, and means including a resistor connected effectively between a point intermediate said pick-up coils and a point intermediate said current responsive devices for passing said derived current through said responsive devices.

6. A radio frequency wattmeter comprising coupling means for connection in series with a power circuit energizing a load, a pair of pick-up coils coupled to said coupling means, a pair of current responsive devices, said pick-up coils and said devices being connected in series so that first currents are induced in said pick-up coils corresponding to currents in said circuit flow in a given direction through said'devices, and means connected to said power circuit and including a series resonant circuit connected to said pick-up coils for deriving from currents flowing in said coupling means a second current which is in phase with the voltage of said line, said second current passing through said responsive devices so as to add to said first current in one or said devices and to oppose said first current in the other of said devices.

amass? a power circuit and coupled to said first inductor,

and a meter connected across the output elements of said thermocouples.

B. A device of the character described in claim 7 in which said resonant impedance means also includes a resistor.

GEORGE H. BRDWN. 

